JPH03281543A - Improved heat shrinkable polyolefin film - Google Patents

Abstract

PURPOSE: To obtain the subject film excellent in burn-through resistance, etc., by compounding a specific amt. of low density branched PE with a copolymer of 1-hexane, etc., and ethylene and extruding the obtained compd. as a film and biaxially stretching this film without crosslinking the same.

CONSTITUTION: Low density branched PE (5-40 wt.%) is compounded with a straight chain copolymer with density of 0.900-0.935 g/cc obtained by copolymerizing 5-20 wt.% of a comonomer of 1-hexene and 1-octene and ethylene, and the obtained compd. is extruded as a film. This film is biaxially stretched at least by 3X in respective directions without being crosslinked beforehand to obtain the objective film.

COPYRIGHT: (C)1991,JPO

Description

[Detailed description of the invention]

[0001] TECHNICAL FIELD OF THE INVENTION [0002] The present invention relates to improvements in methods for producing heat-shrinkable films and the resulting films of certain LLDPE polymers. [0002]

[Conventional technology]

Heat-shrinkable polymer films are generally produced by extruding a polymer from a melt into a film and then orienting the film by stretching under temperature conditions where molecular orientation of the film occurs and the film does not tear. . The film is then cooled in the stretched state, and upon subsequent heating, the film shrinks to return to its original dimensional state. [0003] Irradiation of films has been used to crosslink the film (polymer) prior to stretching, thereby increasing its resistance to tearing during stretching, but this increases the cost of irradiation and the thermal This has the double disadvantage that it is not possible to recycle the scrap generated in the production of shrinkable films by melt processing. [0004] The method of making heat-shrinkable films described in U.S. Pat. No. 3,141,912 does not require irradiation of the film prior to stretching, which improves the industrial utility of certain polymers. Achieved. In the continuous process of this patent, the polymer is extruded as a film in the form of a tube, the tube is rapidly cooled below the orientation temperature range, and reheated to the orientation temperature range, and then while within this temperature range the film of the tube is is stretched biaxially. Biaxial stretching involves (a) the use of internal gas to expand the diameter of the tube to form a larger bubble, and (b) the transverse (tr)
ansv-erse direction) (TD)
and machine direction
(MD) orientation is achieved by advancing the expanded tube at a speed faster than the extrusion speed. Typically stretching is done at least 3x in each direction. The film is then cooled and wound in the cold state so as to retain its heat-shrinkable properties. [0005] U.S. Pat. No. 4,820,557 discloses that one of the layers of the multilayer film is a linear copolymer of ethylene and either 1-octene or 1-hexene; discloses the production of multilayer heat shrinkable films with narrow molecular weight distributions and specific melt flow rates exhibiting densities of 0.935 g/cc or less. The layer also contains this polymer and LD
It is disclosed as a blend with a number of other polymers, including PE. However, the manufacture of heat-shrinkable films in this patent is disclosed to include irradiation of the film prior to stretching. [0006] Japanese Patent Publication No. 60-257.232 (198
5) discloses blends of LDPE and LLDPE mixed with a free radical generator to crosslink the polymer in a film extruder. Films are made by conventional inflation techniques in which bubbles are blown at or near the melting point of the polymer. The film has little or no orientation;
Not heat shrinkable. The resulting film bags are disclosed to have good heat seal strength. [0007] US Pat. No. 4,597,920 discloses that the linear polymer is a copolymer of at least one 08-018 alpha olefin and ethylene, and the copolymer has a molecular weight of 0.1 to 4. have a density of 0 g/cc to 0.940 g/cc, a broad molecular weight distribution as indicated by a stress index of greater than or equal to about 1.3, and two distinct molecules separated by at least 10°C. U.S. Pat. No. 3,141,91, having a microcrystalline melting range (melting point)
Discloses the production of a heat-shrinkable film by the method of No. 2. This patent also states that irradiation of the extruded film prior to stretching is optional. In fact, the existence of two distinct melting ranges in the copolymer makes it possible to produce heat-shrinkable films of ethylene/1-octene copolymers on an industrial basis without crosslinking before stretching. There is. This commercially available film is called Clysar® LLP shrink film.

[0008] This patent includes the disclosure in Table IV of formulating increasing amounts of low density branched polyethylene mixed with the ethylene/1-octene copolymer; as a result, the shrinkage force of the film is reduced. has been reported. This aspect of the disclosure provides that the operation from forming the film by melt pressing is discontinuous.
Stretching of the film of the formulation was reported to have been carried out in a laboratory stretching machine, and in fact did not go beyond laboratory studies. [0009] Despite the industrial success of the ethylene/1-octene copolymer heat-shrinkable films of U.S. Pat. There was a desire to increase the economics of manufacturing this film without incurring the disadvantages of non-melt processable scrap from film manufacture. One method of economization would be to increase the production speed of the film on the same production equipment. The rate of production of this heat-shrinkable film has been limited by the difficulty of achieving the temperature control necessary for orientation of the film within the stretching section of the manufacturing machine. The extrusion speed of the film indicates that there is a uniform elongation of the film, and that the desired machine direction elongation is stably located in the elongation zone of the manufacturing equipment, including that the shape of the bubble is symmetrical. This condition of bubble stability provides conditions for biaxial orientation without tearing of the film upon stretching. If the extrusion rate could be increased without causing ethylene/1-octene elongation and without reducing elongation in the machine direction, large quantities of heat-shrinkable film could be obtained from the same manufacturing machine. In the case of LLDPE, the problems in producing heat-shrinkable films without crosslinking prior to elongation are various. Such production is rarely possible, i.e. crosslinking prior to elongation. The production of heat-shrinkable films from this copolymer without any heat shrinkage appears to have never been reported in the prior art.Since the temperature required for orientation is very close to the melting point of the copolymer, the extrusion rate is low. This proximity between the orientation temperature and the melting point is the reason why the strength of the copolymer is extremely low and the film tends to be unevenly oriented and tear. It is for this reason that U.S. Pat. No. 4,597,920 required an alpha olefin comonomer containing at least 8 carbon atoms along with ethylene, thereby excluding the disclosure of 1-hexene. ]

[Summary of the present invention]

The present invention solves several problems as mentioned above; namely:
The present invention allows LLDPE in which the alpha monoolefin comonomer is 1-hexene to be used to produce heat-shrinkable films without tearing, and LLDPE in which the alpha monoolefin comonomer is 1-octene. The manufacturing speed of the film can be increased significantly without tearing of the film. These improvements are obtained without crosslinking the LLDPE prior to elongation. [0012] More particularly, the present invention provides a method for producing a 0.0. Extruding the copolymer as a film from a linear copolymer having a density of 900 to 0.935 g/cc and stretching the film biaxially by at least 3x in each direction without prior crosslinking. can be described as an improvement to the continuous process for producing heat-shrinkable films, which comprises from 5 to 40% by weight of low-density branched polyethylene (LDPE), based on the weight of the resulting formulation. ) is blended with the copolymer prior to extrusion. [0013] In one embodiment of the invention, when the comonomer is 1-hexene, the addition of LDPE to LLDPE allows the resulting formulation to be stretched uniformly at a reasonable rate and of high quality without tearing. However, neither this particular LLDPE nor LDPE by itself has this capability in the aforementioned process. Other embodiments of the invention provide that the resulting heat-shrinkable film is free of crosslinking and is capable of shrinking by at least 10% in each direction opposite to the direction of biaxial orientation when heated, and 5 to 20% based on the weight of the copolymer with a density of 935 g/cc
a linear copolymer of 1-hexene and ethylene of 60 to 95% by weight, based on the weight of the formulation, together with 5 to 40% by weight of low density branched polyethylene. [0014] In yet another embodiment of the invention, the comonomer is 1-
If octene, the addition of LDPE to LLDPE will cause the resulting heat-shrinkable film to
The extrusion rate of the resulting formulation can be increased to provide increased economics without penalizing the desired properties compared to films of PE itself. [0015]

[Description of preferred embodiment]

The process for producing heat-shrinkable films according to the process of the invention is a conventional process, apart from the addition of LDPE to the LLDPE polymer described and the results achieved as described above. A detailed description of the method is provided in U.S. Pat. No. 4,597,920.
and U.S. Pat. No. 3,141,912;
, No. 116, column 4, lines 1 to 22. [0016] Preferably, the film extrudate in the method used in the present invention is tubular, the elongation of the film is at least 3× in the biaxial direction, and the elongation of the film is preferably at least 3× in the machine direction. 5× and at least 4× in the lateral direction. Preferably ethylene/1 in these same biaxial orientations
- The extrusion rate of formulations containing octene copolymers is
Compared to the maximum extrusion speed achievable without the addition of E, the process according to the invention increases it by at least 5%. In the case of ethylene/1-octene copolymers, the current extrusion speed of 16.5 m/min can be increased to preferably at least 17.5 m/min, and preferably at least 18 m/min. It is preferable that the orientation temperature (orientation temperature) is at least 10° C. lower than the melting point of the film. The copolymer present has a major melting point and a minor melting point.
r) melting point (below the main melting point), it is preferred that the orientation temperature is at least 10° C. below the main melting point. The extrusion temperature is generally in the range of 215-235°C and the melting point (or major melting point) of the polymer blend is generally in the range of 115-130°C. Quenching the extruded film generally involves cooling the film to ambient temperature. The film is then reheated to the orientation temperature for elongation. Since the film has a certain tolerance that allows it to stretch without tearing, the orientation temperature is within a temperature range that is considered to be virtually at least 5°C. The quench/reheat method provides a controlled means to achieve the orientation temperatures required for high production rates. The formulation is not crosslinked prior to elongation, i.e. it is not irradiated after extrusion, nor are free radical generators added to the formulation prior to extrusion to cause crosslinking to be present in the film extrudate. [0017] The LLDPE used in the present invention is the above-mentioned linear copolymer of 1-hexene or 1-octene and ethylene. These copolymers are made by conventional low pressure polymerization techniques using transition metal catalysts, such as those described in U.S. Pat. No. 4,597,920. This LLDPE is mainly 1-
It has short chain branching of hexene or 1-octene comonomer side groups and little to no long chain branching from the polymer backbone. Generally, the copolymer is 0.1 to 4.0
g710 min, preferably a melt index of 0.5 to 2° g710 min, and a melt index of 0.900 to 0.93
5 g/Cm3, preferably 0.910 to 0.930
It has a density of g/Cm3. [0018] In the case of the 1-octene copolymer, the LLDPE preferably has a stress index of 1.3 or higher and two distinct melting points of 128° C. or lower, such that the temperature difference between these melting points is at least 10° C., and the higher melting point is the primary melting point of LLDPE as stated in the aforementioned US patent. In the case of 1-hexene copolymers, LLDPE does not exhibit these distinct melting point ranges, but instead has only one melting point. [0019] The 1-octene content of the copolymer is preferably from 8 to 16% by weight, based on the weight of the copolymer. LLDPE
The preferred proportion of 1-hexene therein is preferably 6 to 15 weight based on the weight of the copolymer. In each case, the ethylene content is 100% of the total weight of the copolymer minus the percentage of comonomer. [0020] The LLDPE may contain slip and antiblock agents such as 1500 to 3000 ppm erucamide and 100 to 2000 ppm powdered silica, and other conventional additives to polymers. These small amounts of additives are present in the formulations with LDPE described in the text.
Contained as a weight ratio of LDPE. [0021] As used in the present invention, the term low density branched polyethylene is often abbreviated in the text as LDPE,
Polymers that are either homopolymers of ethylene or copolymers of ethylene with small amounts of other ethylenically unsaturated comonomers, prepared by conventional high-pressure polymerization processes involving free radical catalysis. means. The resulting polymer is highly branched, ie, has a large number of long chain branches generated by free radical catalysis. LDPE as an ethylene homopolymer is generally 0.910 to 0.930 g/cm
, and preferably 0.915 to 0.925 g/cm
3 density and a melt index of 1.0 to 10 g/10 min, preferably 1.5 to 4.5 g/10 min. If the LDPE is a copolymer, the comonomer generally does not exceed 12% by weight based on the weight of the copolymer. The preferred comonomer is vinyl acetate for reasons of copolymer economics, but many other minor comonomers can be used as recognized by those skilled in the art and still achieve the improvements described herein. LDPE as a homopolymer or copolymer is the LLDP used in the present invention.
It is preferred to have a melting point at least 10°C lower than that of E. If LLDPE has two melting points,
Preferably, the melting point of the LDPE is at least 10'C lower than the higher of the two melting points. The LDPE may also contain small amounts of additives commonly used in LDPE, ie, slip agents and antiblocking agents, which do not interfere with the effectiveness of the LDPE in the formulation. [0022] LD in the formulation for LLDPE used in the present invention
The proportion of PE is selected as an effective amount to provide the improved results described above depending on the particular LLDPE used. This improvement ranges from 5 to 30 or 40% by weight.
Since the LD in the formulation can be realized within the range of
The preferred amount of PE is 10 to 35% by weight, and L
When the comonomer in LDPE is 1-hexene,
20 to 35% or 15 to 25 or 30% by weight of the formulation within this range and the comonomer is 1-
10 to 25% by weight of the formulation if octenes;
or 15 to 30% by weight, these weight percentages being based on the total weight of the formulation. [0023] The LDPE and LLDPE components of the formulations used in the present invention can be dry blended in the desired proportions with the ingredients, generally in the form of moldable granules, by conventional mixing means. The resulting dry blend can then be fed to an extruder to form a film. The uniformity of the formulation can be improved by melt blending the components and additives together and feeding the resulting melt blended forming granules to an extruder where the film is formed. The heat-shrinkable film of the present invention is characterized by uniform shrinkability in biaxial directions resulting from the uniform orientation achieved in the stretching operation. These films are useful for packaging and packaging articles in the manner in which heat-shrinkable films have traditionally been used. Generally, the heat shrinkable films made according to the present invention are used as single layer films having a uniform thickness of 0.013 (0.5 mil) to 0.051 (2.0 mil) mm. These heat-shrinkable films can be irradiated in combination with other materials as needed and/or as a layer in a multilayer structure to achieve specific properties, such as barrier properties not exhibited by a single layer on its own. You can also do it. [0024] Examples of the invention are as follows (parts and percentages are by weight unless otherwise specified). [0025]

Example 1 A blend of 20% by weight LDPE and 80% by weight LLDPE was prepared by dry compounding. LLDPE was a linear copolymer of ethylene and about 11% by weight 1-octene, based on the weight of the copolymer. Each copolymer is 122
and a major and minor melting point of 109° C., a melt index of 1.1 g/10 min, and a density of 0.921 g/cc, and 2000 pprn of nilucamide and 13
It contained 00 ppm of powdered silica. LDPE is 10
Melting point of 9℃, melt index of 1.9g 710 minutes,
and a highly branched homopolymer of ethylene with a density of 0.923 g/cc. Particles of the formulation were fed into an extruder. The formulation was extruded as a tubular film and quenched so that a 0.75 mil (0.02 mm) thick heat-shrinkable film was formed by the conventional continuous process of U.S. Pat. No. 3,141,912. , reheated to the elongation temperature range, biaxially elongated, and cooled in the elongated state. Details of the method are as follows: extrusion temperature 230
℃, quenching temperature 25℃, reheating temperature range (film orientation temperature) about 105 to 110℃, reheated tubular film is blown as a bubble, and the winding is operated at a surface speed higher than the extrusion speed. The degree of elongation obtained using take-up rolls is 4× in the machine direction and 5× in the transverse direction.
Met. Fill' (7) in the bubble generated by introducing air under pressure into the extruded tube
7-pu stress is 16,548 kpa (2,400 psi
)Met. For comparison purposes, the same LLDPE was used alone and processed in the same process. The results of these experiments are shown in the table below. [0026] Artificial LLDPE 80:20 combination of LLDPE and LDPE Press once a series of upper L'1Z minutes upper L-Shu Z is ↓L'37 minutes *) JV contracted isometric film Ω properties - 11111 cloudy degree, % 2.1 2.0 1.9 Glossiness i S os Transparency, % Shrinkage, % (102℃) D D Modulus, Pa D D Tensile strength, P D D Elongation, % D D Coefficient of friction i 0.12 0, 10 0.09 os 0.11 0.09 0.11 Nirmendorf, Tear, D D Extrusion speed of the film of LLDPE itself 16.5 m/
minutes is the maximum speed that can be tolerated without film bubble instability. Table ■ shows that the extrusion speed can be increased by as much as 12% for films of LLDPE/LDPE blends without any significant sacrifice in film properties, even if the elongation in the machine direction is slightly reduced. is not included. Of course, the film winding speed was increased to maintain the same degree of machine direction elongation as the extrusion speed was increased. At fast production rates of heat-shrinkable films, the Tolong process using the film formulation according to the invention is smooth, i.e. without machine downtime due to film tearing, and the slow process using only LLDPE (but with LLDPE
The process appeared to run smoothly at extrusion speeds (highest for 2000). [0027] Heat-shrinkable films produced by the method of the present invention, such as the method described in this example, exhibit burn-through when heat is applied to shrink-wrap an article sealed within the film. (burn-through) irradiation for the purpose of increasing resistance. The heat during heating overheats the parts of the film that are not in contact with the item during heating, and this overheating can melt the film and form holes in the shrink wrap, and these holes are called ``burn-through''. Ru. This irradiation can be carried out in a separate operation from the production of the heat-shrinkable film, as it is possible to collect all scraps from the production of the film, such as trimming edges of the film, etc., prior to irradiation. , the scrap can be recycled through melt processing. [0028] The heat shrinkable film made of LLDPE alone used in this example exhibited burn-through at about 360°F (182°C). The burn-through temperature of the film increased to 395°F (202°C) when exposed to 2 megarads of radiation. The heat-shrinkable film of the formulation used in this example showed a
exhibits a burn-through temperature of 90' F (199 C);
Heat-shrinkable films produced by the method of the present invention thereby provide comparable improvements in burn-through resistance when irradiated. [0029]

Example 2 By the continuous method of US Pat. No. 3,141,912, 1
Single melting point of 22°C, density of 0.918g/cc and 1
．． On a different production line than that used in Example 1, using about 9% by weight linear copolymer of 1-hexene and ethylene with a melt index of 0 g 710 min.
Attempts have been made to form heat-shrinkable films. Although various extrusion temperatures, reheat temperatures, and amounts of biaxial elongation were tried, it was not possible to produce stable bubbles. In general, the films did not appear to be uniformly oriented, as indicated by the thick and thin wall thicknesses appearing in the bubbles. Nichilene/
'1-hexene copolymer was dry blended with 20 wt. 96 LDPE used in Example 1, and then
Stable bubbles and uniform heat-shrinkable films were easily formed as a result of extrusion according to the continuous method of No. 141,912. The working conditions used for the successful operation were as follows; extrusion temperature was 225-230 °C, speed was 2 m/min, and film orientation temperature was 105-1
The temperature was 10° C. and the nofilm hoop stress in the bubble was 11,721 kpa (1,700 psi). To be able to compare the properties of the resulting heat-shrinkable films,
The LLDPE copolymer of Example 1 was worked under the same conditions and the results are shown in Table II. [0030] Table 1 Blend with LDPE of Example 1 (80: Copolymerized ethylene/1-octene ethylene/1-hexene copolymer film thickness 0.032 mm (1,2 mil) 0.029 mm (1 , 1 mil) Tatami IQ Office Gloss Transparency Shrinkage, % (102°C) D D The properties of the film produced from the formulation are ethylene/1-
Advantageously, the properties are comparable to those of films made with octene copolymer alone. The extrusion speed used was believed to be much lower than the highest extrusion speed possible for films made from the formulation. [0031] Results similar to Examples 1 and 2 were obtained using L
Small and large amounts of LDPE blended with LDPE copolymers
can be obtained using The relatively high level of LDPE used in the examples also provides economy in the resulting films in that LDPE is less expensive than LLDPE copolymers. [0032] Because the present invention is capable of numerous and wide variety of embodiments without departing from the spirit and scope of the invention, the present invention encompasses specific embodiments other than as set forth in the appended claims. It should be understood that this is not limited to. [0033] Main features and aspects of the present invention are as follows. [0034] 1. 0,9 by copolymerization of ethylene with a comonomer selected from the group consisting of 1-hexene and 1-octene in an amount of 5 to 20% by weight based on the weight of the copolymer.
From a linear copolymer having a density of 0.00 to 0.935 g/cc, the copolymer is extruded as a film and the resulting film is subjected to at least 3 5 to 40% by weight, based on the weight of the resulting formulation, of the copolymer prior to extrusion in a continuous process for producing heat-shrinkable films by stretching in the direction of An improvement method characterized by combining with. [0035] 2. The comonomer is 1-hexene, and the improvement obtained as a result of blending the branched polyethylene and the linear copolymer is
1 above, the ability to carry out a uniform stretching process without tearing the film, and the proportion of branched polyethylene present within the range of 5 to 40% by weight is selected to provide this improvement. The method described in. [0036] 3. The comonomer is 1-octene, and the improvement obtained as a result of blending the branched polyethylene and the linear copolymer is
The ability to increase the extrusion rate without increasing tearing of the film during the stretching process and the proportion of branched polyethylene present within the range of 5 to 40% by weight was selected to provide this improvement. The method according to 1 above. [0037] 4. The method according to 1 above, further comprising irradiating the heat-shrinkable film. [0038] 5. 60 to 95% by weight based on the weight of the formulation, 5 to 20% by weight based on the weight of the copolymer, having a density of 0.900 to 0.935 g/cc.
- a biaxially oriented heat-shrinkable film of a blend of a linear copolymer of hexene and ethylene with 5 to 40% by weight, based on the weight of the blend, of low-density branched polyethylene; the film is free of crosslinking, and upon application of heat it forms at least one crosslink in each direction opposite to the direction of orientation.
A film that can shrink by 0%.

6. 0.90 by copolymerization of ethylene with a comonomer selected from the group consisting of 1-hexene and 1-octene in an amount of 5 to 20% by weight based on the weight of the copolymer.
From a linear copolymer having a density of 0 to 935 g/cc, the copolymer is extruded as a tubular film, the film is rapidly cooled to a temperature below the orientation temperature of the film, and the film is brought to the orientation temperature. reheating and expanding the tube of film to form a bubble to obtain an elongation of at least 3 times the length of the original film in each biaxial direction; and By advancing the film at a speed faster than the extrusion speed, the film is stretched biaxially while at the orientation temperature, the biaxial stretching occurs without prior cross-linking, and the heat-shrinkable properties In a continuous process for producing heat-shrinkable films by cooling the stretched film so as to hold the copolymer at 5% by weight based on the weight of the formulation prior to extrusion.
to 40% by weight of low density branched polyethylene which causes tearing of the film upon orientation when the comonomer in the copolymer is 1-octene. or, when the comonomer in the copolymer is 1-hexene, allows heat-shrinkable films to be produced with uniform elongation and without tearing. An improvement method characterized by an effective amount. [0040] 7. In the method described in 6 above, the amount of the I density branched polyethylene is 5 to 30% by weight. [0041] 8. In the method described in 6 above, when the monomer is 1-hexene, the amount of low density branched polyethylene is 20
and 35% by weight. [0042] 9. In the method described in 6 above, when the monomer is 1-octene, the amount of low density branched polyethylene is 15
The content must be between 30% and 30% by weight.

Claims (3)

[Claims] Claim 1: 5 to 20% based on the weight of the copolymer
% by weight of a linear copolymer having a density of 0.900 to 0.935 g/cc by copolymerization of ethylene with a comonomer selected from the group consisting of 1-hexene and 1-octene. In a continuous process for producing heat-shrinkable films by extruding the polymer as a film and biaxially stretching the resulting film by at least 3x in each direction without prior crosslinking, the resulting formulation An improved process characterized in that from 5 to 40% by weight, based on the weight of the copolymer, is blended with the copolymer prior to extrusion. Claim 2: 60 to 95, based on the weight of the formulation.
5 to 20% by weight, based on the weight of the copolymer, of a linear copolymer of 1-hexene and ethylene having a density of 0.900 to 0.935 g/cc;
A biaxially oriented heat-shrinkable film of a blend with 5 to 40% by weight, based on the weight of the blend, of low density branched polyethylene, the film comprising no crosslinks;
and a film capable of shrinking by at least 10% in each direction opposite the direction of orientation upon application of heat. Claim 3: 5 to 20% based on the weight of the copolymer
% by weight of a linear copolymer having a density of 0.900 to 0.935 g/cc by copolymerization of ethylene with a comonomer selected from the group consisting of 1-hexene and 1-octene. extruding the polymer as a tubular film, rapidly cooling the film to a temperature below the orientation temperature of the film, and reheating the film to the orientation temperature;
Expanding the tube of film to form a bubble such that an elongation of at least 3× the length of the original film in each biaxial direction is obtained, and lowering the expanded tube of film at an extrusion rate. Advancing at a high speed also ensures that the film is biaxially stretched while at the orientation temperature, that the biaxial stretching occurs without prior cross-linking, and that the heat-shrinkable properties are retained. In a continuous process for producing heat-shrinkable films by cooling the stretched film, the copolymer is added to a low-density film in an amount ranging from 5 to 40% by weight, based on the weight of the formulation, prior to extrusion. When combined with branched polyethylene, the branched polyethylene allows extrusion rates to be increased without tearing the film during orientation when the comonomer in the copolymer is 1-octene. , or when the comonomer in the copolymer is 1-hexene, characterized in that it is in an amount effective to enable a heat-shrinkable film to be produced with uniform stretching without tearing. Improvement method.